High-frequency signal transmission line and electronic device

ABSTRACT

A high-frequency signal transmission line includes an element, a linear signal line provided at the element and including a first end and a second end, and at least one ground conductor provided at the element and extending along the signal line. The element includes stacked insulating layers. The ground conductor is positioned opposite to the signal line with the insulating layer positioned therebetween. The ground conductor is a contiguous conductor. The signal line, the ground conductor, and the element generate a characteristic impedance. The signal line includes a first section and a second section. The first section is an uninterrupted section generating a characteristic impedance greater than or equal to a first characteristic impedance at the first end and including the first end. The second section generates a characteristic impedance less than the first characteristic impedance and is adjacent to the first section. The second section is longer than the first section. The signal line is wider in the second section than in the first section.

This application is based on International Application No.PCT/JP2014/084523 filed on Dec. 26, 2014, and Japanese PatentApplication No. 2014-19079 filed on Feb. 4, 2014, the entire content ofeach of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to high-frequency signal transmissionlines and electronic devices, more particularly to a high-frequencysignal transmission line including a signal line and ground conductors,and an electronic device including the same.

2. Description of the Related Art

As a conventional high-frequency signal transmission line, ahigh-frequency signal line described in, for example, InternationalPublication No. 2012/074101 is known. This high-frequency signal lineincludes a dielectric element, a signal line, a first ground conductor,and a second ground conductor. The signal line, the first groundconductor, and the second ground conductor are provided at thedielectric element. Moreover, the signal line is vertically sandwichedbetween the first ground conductor and the second ground conductor. Thatis, the signal line, the first ground conductor, and the second groundconductor form a stripline structure. Further, the second groundconductor includes a plurality of openings overlapping the signal line.Such a high-frequency signal line having the openings provided in thesecond ground conductor renders it possible to reduce capacitancegenerated between the second ground conductor and the signal line, sothat the dielectric element can be rendered thinner by providing thesecond ground conductor and the signal line closer to each other.

There has been demand to further reduce insertion loss in thehigh-frequency signal line described in International Publication No.2012/074101.

SUMMARY OF THE INVENTION

A first aspect of various preferred embodiments of the present inventionis directed to a high-frequency signal transmission line including anelement, a linear signal line provided at the element and including afirst end and a second end, and at least one ground conductor providedat the element and extending along the signal line. The element includesstacked insulating layers. The ground conductor is positioned oppositeto the signal line with the insulating layer positioned therebetween.The ground conductor is a contiguous conductor. The signal line, theground conductor, and the element generate a characteristic impedance.The signal line includes a first section and a second section. The firstsection is an uninterrupted section generating a characteristicimpedance greater than or equal to a first characteristic impedance atthe first end and including the first end. The second section generatesa characteristic impedance less than the first characteristic impedanceand is adjacent to the first section. The second section is longer thanthe first section. The signal line is wider in the second section thanin the first section.

A second aspect of various preferred embodiments of the presentinvention is directed to a high-frequency signal transmission lineincluding an element, a linear signal line provided at the element andincluding a first end and a second end, and at least one groundconductor provided at the element and extending along the signal line.The element includes stacked insulating layers. The ground conductor ispositioned opposite to the signal line with the insulating layerpositioned therebetween. The ground conductor includes a plurality ofopenings arranged along the signal line. The signal line, the groundconductor, and the element generate a characteristic impedance. Thesignal line includes a first section and a second section. The firstsection is an uninterrupted section generating a characteristicimpedance greater than or equal to a first characteristic impedance atthe first end and including the first end. The second section generatesa characteristic impedance less than the first characteristic impedanceand is adjacent to the first section. The second section is longer thanthe first section. The openings are arranged in descending order ofoverlapping area with the signal line in the second section toward afarthest end from the first section.

A third aspect of various preferred embodiments of the present inventionis directed to an electronic device including a high-frequency signaltransmission line, and a housing accommodating the high-frequency signaltransmission line. The high-frequency signal transmission line includesan element, a linear signal line provided at the element and including afirst end and a second end, and at least one ground conductor providedat the element and extending along the signal line. The element includesstacked insulating layers. The ground conductor is positioned oppositeto the signal line with the insulating layer positioned therebetween.The ground conductor is a contiguous conductor. The signal line, theground conductor, and the element generate a characteristic impedance.The signal line includes a first section and a second section. The firstsection is an uninterrupted section generating a characteristicimpedance greater than or equal to a first characteristic impedance atthe first end and including the first end. The second section generatesa characteristic impedance less than the first characteristic impedanceand is adjacent to the first section. The second section is longer thanthe first section. The signal line is wider in the second section thanin the first section.

A fourth aspect of various preferred embodiments of the presentinvention is directed to an electronic device including a high-frequencysignal transmission line, and a housing accommodating the high-frequencysignal transmission line. The high-frequency signal transmission lineincludes an element, a linear signal line provided at the element andincluding a first end and a second end, and at least one groundconductor provided at the element and extending along the signal line.The element includes stacked insulating layers. The ground conductor ispositioned opposite to the signal line with the insulating layerpositioned therebetween. The ground conductor includes a plurality ofopenings arranged along the signal line. The signal line, the groundconductor, and the element generate a characteristic impedance. Thesignal line includes a first section and a second section. The firstsection is an uninterrupted section generating a characteristicimpedance greater than or equal to a first characteristic impedance atthe first end and including the first end. The second section generatesa characteristic impedance less than the first characteristic impedanceand is adjacent to the first section. The second section is longer thanthe first section. The openings are arranged in descending order ofoverlapping area with the signal line in the second section toward afarthest end from the first section.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external oblique view of a high-frequency signaltransmission line 10 according to a preferred embodiment of the presentinvention.

FIG. 2 is an exploded view of a dielectric element 12 of thehigh-frequency signal transmission line 10 in FIG. 1.

FIG. 3 is a cross-sectional structure view of the high-frequency signaltransmission line 10 in FIG. 1.

FIG. 4 is another cross-sectional structure view of the high-frequencysignal transmission line 10.

FIG. 5A is an oblique view illustrating a connector 100 b and aconnecting portion 12 c.

FIG. 5B is a cross-sectional view of the connector 100 b.

FIG. 5C is a graph showing the characteristic impedance of thehigh-frequency signal transmission line 10.

FIG. 6A illustrates an electronic device 200 provided with thehigh-frequency signal transmission line 10 as viewed in a plan view inthe y-axis direction.

FIG. 6B illustrates the electronic device 200 provided with thehigh-frequency signal transmission line 10 as viewed in a plan view inthe z-axis direction.

FIG. 7A is an exploded view of a dielectric element 12 of ahigh-frequency signal transmission line 10 a.

FIG. 7B is a graph showing simulation results.

FIG. 8A is an exploded view of a dielectric element 12 of ahigh-frequency signal transmission line 10 b.

FIG. 8B is a cross-sectional structure view of the high-frequency signaltransmission line 10 b taken along line A-A in FIG. 8A.

FIG. 8C is a graph showing the characteristic impedance of thehigh-frequency signal transmission line 10 b.

FIG. 9A is an exploded view of a dielectric element 12 of ahigh-frequency signal transmission line 10 c.

FIG. 9B is a graph showing the characteristic impedance of thehigh-frequency signal transmission line 10 c.

FIG. 10A is a graph showing the reflection characteristic of a thirdmodel.

FIG. 10B is a graph showing the pass characteristic of the third model.

FIG. 11A is a graph showing the reflection characteristic of a fourthmodel.

FIG. 11B is a graph showing the pass characteristic of the fourth model.

FIG. 12A is a graph showing the reflection characteristic of a fifthmodel.

FIG. 12B is a graph showing the pass characteristic of the fifth model.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, high-frequency signal transmission line according topreferred embodiments of the present invention, along with an electronicdevice using the same, will be described with reference to the drawings.

The configuration of the high-frequency signal transmission line 10according to the present preferred embodiment will be described below.FIG. 1 is an external oblique view of the high-frequency signaltransmission line 10 according to the present preferred embodiment. FIG.2 is an exploded view of a dielectric element 12 of the high-frequencysignal transmission line 10 in FIG. 1. FIG. 3 is a cross-sectionalstructure view of the high-frequency signal transmission line 10 inFIG. 1. FIG. 4 is another cross-sectional structure view of thehigh-frequency signal transmission line 10. FIG. 5A is an oblique viewillustrating a connector 100 b and a connecting portion 12 c. FIG. 5B isa cross-sectional view of the connector 100 b. FIG. 5C is a graphshowing the characteristic impedance of the high-frequency signaltransmission line 10. In FIG. 5C, the vertical axis represents thecharacteristic impedance, and the horizontal axis represents thex-coordinate. In FIGS. 1 through 4, the stacking direction of thehigh-frequency signal transmission line 10 is defined as the z-axisdirection. In addition, the longitudinal direction of the high-frequencysignal transmission line 10 is defined as an x-axis direction, and thedirection perpendicular to the x-axis and z-axis directions are definedas the y-axis direction.

Furthermore, the length refers below to the dimension of the signal line20 and other components in the x-axis direction. Also, the width refersbelow to the dimension of the signal line 20 and other components in they-axis direction. In addition, the thickness refers below to thedimension of the dielectric sheet 18 and other components in the z-axisdirection.

The high-frequency signal transmission line 10 preferably is used to,for example, connect two high-frequency circuits in an electronic devicesuch as a cellphone. The high-frequency signal transmission line 10includes the dielectric element 12, external terminals 16 a and 16 b, asignal line 20, ground conductors 22 and 24, via-hole conductors b1, b2,B1, B2, B3, and B4, a connector 100 a, and the connector 100 b, as shownin FIGS. 1 through 3.

The dielectric element 12 is in the form of a strip extending in thex-axis direction when viewed in a plan view in the z-axis direction, andincludes a line portion 12 a, a connecting portion 12 b, and theconnecting portion 12 c. The dielectric element 12 is a stack formed bystacking a protective layer 14 and dielectric sheets 18 a, 18 b, and 18c, in this order from the positive side to the negative side in thez-axis direction, as shown in FIG. 2. In the following, principalsurfaces of the dielectric element 12 and the dielectric sheets 18 a to18 c that are located on the positive side in the z-axis direction willbe referred to as top surfaces, and principal surfaces of the dielectricelement 12 and the dielectric sheets 18 a to 18 c that are located onthe negative side in the z-axis direction will be referred to as bottomsurfaces.

The line portion 12 a extends in the x-axis direction. The connectingportion 12 b is connected to the end of the line portion 12 a on thenegative side in the x-axis direction. The connecting portion 12 c isconnected to the end of the line portion 12 a on the positive side inthe x-axis direction. The connecting portions 12 b and 12 c are areas onwhich the connectors 100 a and 100 b to be described later arerespectively mounted. The width of each of the connecting portions 12 band 12 c is equal to the width of the line portion 12 a.

The dielectric sheets 18 a to 18 c, when viewed in a plan view in thez-axis direction, extend in the x-axis direction and have the samestrip-shaped structure as the dielectric element 12. The dielectricsheets 18 a to 18 c are made of a flexible thermoplastic resin such aspolyimide or liquid crystal polymer. The dielectric sheet 18 a has athickness T1 equal or substantially equal to a thickness T2 of thedielectric sheet 18 b, as shown in FIG. 4. After the stacking of thedielectric sheets 18 a to 18 c, the thicknesses T1 and T2 preferablyare, for example, within the range of from about 50 μm to about 300 μm.In the present preferred embodiment, both the thicknesses T1 and T2preferably are about 150 μm.

Furthermore, the dielectric sheet 18 a includes a line portion 18 a-aand connecting portions 18 a-b and 18 a-c. The dielectric sheet 18 bincludes a line portion 18 b-a and connecting portions 18 b-b and 18b-c. The dielectric sheet 18 c includes a line portion 18 c-a andconnecting portions 18 c-b and 18 c-c. The line portions 18 a-a, 18 b-a,and 18 c-a constitute the line portion 12 a. The connecting portions 18a-b, 18 b-b, and 18 c-b constitute the connecting portion 12 b. Theconnecting portions 18 a-c, 18 b-c, and 18 c-c constitute the connectingportion 12 c.

The external terminal 16 a is a rectangular or substantially rectangularconductor provided near the center on the top surface of the connectingportion 18 a-b, as shown in FIGS. 1 and 2. The external terminal 16 b isa rectangular or substantially rectangular conductor provided near thecenter on the top surface of the connecting portion 18 a-c, as shown inFIGS. 1 and 2. The external terminals 16 a and 16 b are made of a metalmaterial mainly composed of silver or copper and having a low specificresistance. In addition, the top surfaces of the external terminals 16 aand 16 b are plated with gold, for example.

The signal line 20 is a linear conductor provided in the dielectricelement 12 and extending on the top surface of the dielectric sheet 18 bin the x-axis direction, as shown in FIG. 2. Accordingly, the signalline 20 has an end located on the negative side in the x-axis direction(first end) and an end located on the positive side in the x-axisdirection (second end). The ends of the signal line 20 in the x-axisdirection respectively overlap the external terminals 16 a and 16 b whenviewed in a plan view in the z-axis direction. The signal line 20 ismade of a metal material mainly composed of silver or copper and havinga low specific resistance. The signal line 20 having such a structuretransmits a high-frequency signal. The width of the signal line 20 willbe described later.

The ground conductor 22 is provided in the dielectric element 12, moreprecisely, on the top surface of the dielectric sheet 18 a, so as to belocated on the positive side in the z-axis direction relative to thesignal line 20, as shown in FIG. 2. The ground conductor 22 extends inthe x-axis direction on the top surface of the dielectric sheet 18 a,and is opposite to the signal line 20 with the dielectric sheet 18 apositioned therebetween. The ground conductor 22 is made of a metalmaterial mainly composed of silver or copper and having a low specificresistance.

Furthermore, the ground conductor 22 includes a line portion 22 a andterminal portions 22 b and 22 c. The line portion 22 a is provided onthe top surface of the line portion 18 a-a and extends in the x-axisdirection. The line portion 22 a has essentially no openings providedtherein. That is, the line portion 22 a is a so-called contiguousconductor extending uninterruptedly in the x-axis direction along thesignal line 20. However, the line portion 22 a may have some minuteholes that unintentionally occurred during the production of thehigh-frequency signal transmission line 10. The terminal portion 22 bpreferably is a rectangular or substantially rectangular framesurrounding the external terminal 16 a on the top surface of theconnecting portion 18 a-b. The terminal portion 22 b is connected to theend of the line portion 22 a on the negative side in the x-axisdirection. The terminal portion 22 c preferably is a rectangular orsubstantially rectangular frame surrounding the external terminal 16 bon the top surface of the connecting portion 18 a-c. The terminalportion 22 c is connected to the end of the line portion 22 a on thepositive side in the x-axis direction. The ground conductor 22 havingsuch a structure is subjected to application of a ground potential.

The ground conductor 24 is provided in the dielectric element 12, moreprecisely, on the top surface of the dielectric sheet 18 c, so as to belocated on the negative side in the z-axis direction relative to thesignal line 20, as shown in FIG. 2. The ground conductor 24 extends inthe x-axis direction on the top surface of the dielectric sheet 18 c,and is opposite to the signal line 20 with the dielectric sheet 18 bpositioned therebetween. That is, the ground conductor 24 is opposite tothe ground conductor 22 with the signal line 20 positioned therebetween.The ground conductor 24 is made of a metal material mainly composed ofsilver or copper and having a low specific resistance.

Furthermore, the ground conductor 24 includes a line portion 24 a andterminal portions 24 b and 24 c. The line portion 24 a is provided onthe top surface of the line portion 18 c-a and extends in the x-axisdirection. The line portion 24 a has essentially no openings providedtherein. That is, the line portion 24 a is a so-called contiguousconductor extending uninterruptedly in the x-axis direction along thesignal line 20. However, the line portion 24 a may have some minuteholes that unintentionally occurred during the production of thehigh-frequency signal transmission line 10. The terminal portion 24 bpreferably is a rectangular or substantially rectangular frame on thetop surface of the connecting portion 18 c-b. The terminal portion 24 bis connected to the end of the line portion 24 a on the negative side inthe x-axis direction. The terminal portion 24 c preferably is arectangular or substantially rectangular frame on the top surface of theconnecting portion 18 c-c. The terminal portion 24 c is connected to theend of the line portion 24 a on the positive side in the x-axisdirection. The ground conductor 24 having such a structure is subjectedto application of a ground potential.

In this manner, the signal line 20 is sandwiched between the groundconductors 22 and 24 with the dielectric sheets 18 a and 18 bintervening on the opposite sides in the z-axis direction. That is, thesignal line 20 and the ground conductors 22 and 24 form a tri-platestripline structure. Moreover, the distance between the signal line 20and the ground conductor 22 preferably is equal or approximately equalto the thickness T1 of the dielectric sheet 18 a, e.g., about 50 μm toabout 300 μm, as shown in FIG. 4. In the present preferred embodiment,the distance between the signal line 20 and the ground conductor 22preferably is about 150 μm, for example. Further, the distance betweenthe signal line 20 and the ground conductor 24 preferably isapproximately equal to the thickness T2 of the dielectric sheet 18 b,e.g., about 50 μm to about 300 μm, as shown in FIG. 4. In the presentpreferred embodiment, the distance between the signal line 20 and theground conductor 24 preferably is about 150 μm, for example.

The via-hole conductor b1 passes through the connecting portion 18 a-bof the dielectric sheet 18 a in the z-axis direction, so as to connectthe external terminal 16 a and the end of the signal line 20 on thenegative side in the x-axis direction. The via-hole conductor b2 passesthrough the connecting portion 18 a-c of the dielectric sheet 18 a inthe z-axis direction, so as to connect the external terminal 16 b andthe end of the signal line 20 on the positive side in the x-axisdirection. Accordingly, the signal line 20 is connected between theexternal terminals 16 a and 16 b. The via-hole conductors b1 and b2 aremade of a metal material mainly composed of silver or copper and havinga low specific resistance.

The via-hole conductors B1 and B2 respectively pass through the lineportion 18 a-a of the dielectric sheet 18 a and the line portion 18 b-aof the dielectric sheet 18 b in the z-axis direction, and when viewed ina plan view in the z-axis direction, the via-hole conductors B1 and B2are located on the positive side in the y-axis direction relative to thesignal line 20. The via-hole conductors B1 and B2 are provided in aplurality respectively in the line portions 18 a-a and 18 b-a, so as tobe arranged in lines in the x-axis direction. Moreover, the via-holeconductors B1 and B2 are connected in pairs, each constituting a singlevia-hole conductor, thus connecting the ground conductor 22 and theground conductor 24. The via-hole conductors B1 and B2 are made of ametal material mainly composed of silver or copper and having a lowspecific resistance.

The via-hole conductors B3 and B4 respectively pass through the lineportion 18 a-a of the dielectric sheet 18 a and the line portion 18 b-aof the dielectric sheet 18 b in the z-axis direction, and when viewed ina plan view in the z-axis direction, the via-hole conductors B3 and B4are located on the negative side in the y-axis direction relative to thesignal line 20. The via-hole conductors B3 and B4 are provided in aplurality respectively in the line portions 18 a-a and 18 b-a, so as tobe arranged in lines in the x-axis direction. Moreover, the via-holeconductors B3 and B4 are connected in pairs, each constituting a singlevia-hole conductor, thus connecting the ground conductor 22 and theground conductor 24. The via-hole conductors B3 and B4 are made of ametal material mainly composed of silver or copper and having a lowspecific resistance.

The protective layer 14 covers approximately the entire top surface ofthe dielectric sheet 18 a. Accordingly, the ground conductor 22 iscovered by the protective layer 14. The protective layer 14 is made of,for example, a flexible resin such as a resist material.

Furthermore, the protective layer 14 includes a line portion 14 a andconnecting portions 14 b and 14 c, as shown in FIG. 2. The line portion14 a covers approximately the entire top surface of the line portion 18a-a, including the line portion 22 a.

The connecting portion 14 b is connected to the end of the line portion14 a on the negative side in the x-axis direction, and covers the topsurface of the connecting portion 18 a-b. The connecting portion 14 bhas openings Ha, Hb, Hc, and Hd provided therein. The opening Ha is arectangular or substantially rectangular opening provided at the centerof the connecting portion 14 b. The external terminal 16 a is exposed tothe outside from the opening Ha. The opening Hb is a rectangular orsubstantially rectangular opening provided on the positive side in they-axis direction relative to the opening Ha. The opening Hc is arectangular or substantially rectangular opening provided on thenegative side in the y-axis direction relative to the opening Ha. Theopening Hd is a rectangular or substantially rectangular openingprovided on the negative side in the y-axis direction relative to theopening Ha. The terminal portion 22 b is exposed to the outside from theopenings Hb to Hd so as to define and function as an external terminal.

The connecting portion 14 c is connected to the end of the line portion14 a on the positive side in the x-axis direction, and covers the topsurface of the connecting portion 18 a-c. The connecting portion 14 chas openings He, Hf, Hg, and Hh provided therein. The opening He is arectangular or substantially rectangular opening provided at the centerof the connecting portion 14 c. The external terminal 16 b is exposed tothe outside from the opening He. The opening Hf is a rectangular orsubstantially rectangular opening provided on the positive side in they-axis direction relative to the opening He. The opening Hg is arectangular or substantially rectangular opening provided on thepositive side in the x-axis direction relative to the opening He. Theopening Hh is a rectangular or substantially rectangular openingprovided on the negative side in the y-axis direction relative to theopening He. The terminal portion 22 c is exposed to the outside from theopenings Hf to Hh so as to define and function as an external terminal.

The connectors 100 a and 100 b are mounted on the top surfaces of theconnecting portions 12 b and 12 c, respectively. The connectors 100 aand 100 b have the same configuration, and therefore, only theconfiguration of the connector 100 b will be described below as anexample.

The connector 100 b includes a connector body 102, external terminals104 and 106, a center conductor 108, and an external conductor 110, asshown in FIGS. 1, 5A, and 5B. The connector body 102 includes arectangular or substantially rectangular plate-shaped portion and acylindrical portion protruding therefrom toward the positive side in thez-axis direction, and is made of an insulating material such as resin.

The external terminal 104 is positioned on the surface of theplate-shaped portion that is located on the negative side in the z-axisdirection, so as to face the external terminal 16 b. The externalterminal 106 is positioned on the surface of the plate-shaped portionthat is located on the negative side in the z-axis direction, so as tocorrespond to the terminal portion 22 c exposed from the openings Hf toHh.

The center conductor 108 is positioned at the center of the cylindricalportion of the connector body 102, and is connected to the externalterminal 104. The center conductor 108 is a signal terminal to/fromwhich a high-frequency signal is inputted/outputted. The externalconductor 110 is positioned on the inner circumferential surface of thecylindrical portion of the connector body 102, and is connected to theexternal terminal 106. The external conductor 110 is a ground terminalto be kept at a ground potential.

The connector 100 b having such a structure is mounted on the topsurface of the connecting portion 12 c, such that the external terminal104 is connected to the external terminal 16 b, and the externalterminal 106 is connected to the terminal portion 22 c. As a result, thesignal conductor 20 is electrically connected to the center conductor108. In addition, the ground conductors 22 and 24 are electricallyconnected to the external conductor 110.

Incidentally, to reduce insertion loss, the high-frequency signaltransmission line 10 preferably has a structure will be described below.Specifically, the signal line 20 includes sections A1 and A2, as shownin FIG. 2. The section A1 is an uninterrupted section including the endof the signal line 20 on the negative side in the x-axis direction andgenerating a characteristic impedance greater than or equal to acharacteristic impedance Z1 (e.g., about 50Ω) at the end of the signalline 20 on the negative side in the x-axis direction, as shown in FIG.5C. More specifically, the section A1 extends toward the positive sidein the x-axis direction starting from the end of the signal line 20 onthe negative side in the x-axis direction. In the present preferredembodiment, the characteristic impedance generated in the section A1 isuniform at the characteristic impedance Z1.

The section A2 is adjacent to the section A1, and generates acharacteristic impedance less than the characteristic impedance Z1, asshown in FIG. 5C. More specifically, the section A2 extends toward thepositive side in the x-axis direction starting from the end of thesection A1 on the positive side in the x-axis direction, and includesthe end of the signal line 20 on the positive side in the x-axisdirection. In addition, the section A2 is longer than the section A1.

Furthermore, the section A2 includes an impedance changing section a1and an impedance unchanging section a2. The impedance changing sectiona1 is adjacent to the section A1, and extends toward the positive sidein the x-axis direction starting from the end of the section A1 on thepositive side in the x-axis direction. In the impedance changing sectiona1, the characteristic impedance decreases toward the farthest end fromthe section A1 (i.e., toward the end on the positive side in the x-axisdirection), as shown in FIG. 5C. The impedance unchanging section a2 isadjacent to the impedance changing section a1, and extends toward thepositive side in the x-axis direction starting from the end of theimpedance changing section a1 on the positive side in the x-axisdirection. The impedance unchanging section a2 generates acharacteristic impedance Z10 (e.g., about 30Ω), which is essentiallyconstant, as shown in FIG. 5C. The impedance unchanging section a2includes the end of the signal line 20 on the positive side in thex-axis direction. Accordingly, the characteristic impedance at the endof the signal line 20 on the positive side in the x-axis direction isequal or substantially equal to the characteristic impedance Z10.

Furthermore, the length of the impedance changing section a1 ispreferably greater than or equal to about ⅕ of the wavelength of thehigh-frequency signal to be transmitted through the signal line 20, morepreferably greater than or equal to about ¼ of the wavelength of thehigh-frequency signal. In addition, the length of the impedance changingsection a1 is preferably less than or equal to twice the wavelength ofthe high-frequency signal to be transmitted through the signal line 20.

In the high-frequency signal transmission line 10, the width of thesignal line 20 varies between the sections with a view to generating thecharacteristic impedance. More specifically, in the section A1, thesignal line 20 has a width w1 (e.g., about 100 μm). In the impedanceunchanging section a2, the signal line 20 has a width w2 (e.g., about300 μm) greater than the width w1. In this manner, in the section A1,the signal line 20 is relatively narrow, and therefore, the area of thesignal line 20 that overlaps the ground conductors 22 and 24 per unitlength is relatively small. Accordingly, in the section A1, thecapacitance generated between the signal line 20 and the groundconductors 22 and 24 per unit length is relatively low, so that thecharacteristic impedance Z1 of the high-frequency signal transmissionline becomes relatively high. On the other hand, in the impedanceunchanging section a2, the signal line 20 is relatively wide, andtherefore, the area of the signal line 20 that overlaps the groundconductors 22 and 24 per unit length is relatively large. Accordingly,in the impedance unchanging section a2, the capacitance generatedbetween the signal line 20 and the ground conductors 22 and 24 per unitlength is relatively high, so that the characteristic impedance Z1 ofthe high-frequency signal transmission line becomes relatively low.

Furthermore, the signal line 20 has the width w1 at the end of theimpedance changing section a1 on the negative side in the x-axisdirection, and also has the width w2 at the end of the impedancechanging section a1 on the positive side in the x-axis direction. Thatis, in the impedance changing section a1, the width of the signal line20 increases from the negative side in the x-axis direction toward thepositive side (i.e., toward the farthest end from the section A1). Inaddition, in the impedance changing section a1, the width of the signalline 20 changes continuously. The wording “the width of the signal line20 changes continuously” herein is intended to mean that the width doesnot change in a discontinuous, stepped manner. As a result, thecharacteristic impedance generated in the impedance changing section a1continuously decreases from the characteristic impedance Z1 to thecharacteristic impedance Z10 from the negative side in the x-axisdirection toward the positive side.

In this manner, the characteristic impedance of the high-frequencysignal transmission line 10 is generated by the signal line 20, theground conductors 22 and 24, and the dielectric element 12. Accordingly,the high-frequency signal transmission line 10 does not use a circuit,such as a balun, which does not include the signal line 20, the groundconductors 22 and 24, and the dielectric element 12, for the purpose ofchanging the characteristic impedance generated in the impedancechanging section a1.

The high-frequency signal transmission line 10 is used in a manner aswill be described below. FIG. 6A illustrates an electronic device 200provided with the high-frequency signal transmission line 10 as viewedin a plan view in the y-axis direction. FIG. 6B illustrates theelectronic device 200 provided with the high-frequency signaltransmission line 10 as viewed in a plan view in the z-axis direction.

The electronic device 200 includes the high-frequency signaltransmission line 10, circuit boards 202 a and 202 b, receptacles 204 aand 204 b, a battery pack 206, and a housing 210.

The housing 210 accommodates the high-frequency signal transmission line10, the circuit boards 202 a and 202 b, the receptacles 204 a and 204 b,and the battery pack 206. The circuit board 202 a is provided with, forexample, a transmitter or receiver circuit including an antenna. Thecircuit board 202 b is provided with, for example, a feed circuit. Thebattery pack 206 is, for example, a lithium-ion secondary battery, andthe surface thereof is wrapped by a metal cover. The circuit board 202a, the battery pack 206, and the circuit board 202 b are arranged inthis order, from the negative side to the positive side in the x-axisdirection.

The receptacles 204 a and 204 b are provided on the principal surfacesof the circuit boards 202 a and 202 b, respectively, on the negativeside in the z-axis direction. The receptacles 204 a and 204 b areconnected to the connectors 100 a and 100 b, respectively. As a result,high-frequency signals to be transmitted between the circuit boards 202a and 202 b at a frequency of, for example, from 600 MHz to 6 GHz (inthe present preferred embodiment, 2 GHz) are applied to the centerconductors 108 of the connectors 100 a and 100 b via the receptacles 204a and 204 b, respectively. Moreover, the external conductors 110 of theconnectors 100 a and 100 b are kept at a ground potential by the circuitboards 202 a and 202 b and the receptacles 204 a and 204 b. Thus, thehigh-frequency signal transmission line 10 connects the circuit boards202 a and 202 b both electrically and physically.

Here, the top surface of the dielectric element 12 (more precisely, thetop surface of the protective layer 14) is in contact with the batterypack 206, as shown in FIG. 6A. The top surface of the dielectric element12 and the battery pack 206 are fixed by an adhesive or the like. Thetop surface of the dielectric element 12 is a principal surface locatedon the ground conductor 22 side relative to the signal line 20.Accordingly, the ground conductor 22, which is in a contiguous form,i.e., the ground conductor 22 extends uninterruptedly in the x-axisdirection, is located between the signal line 20 and the battery pack206.

A non-limiting example of a method for producing the high-frequencysignal transmission line 10 will be described below with reference toFIG. 2. In the following, an example where one high-frequency signaltransmission line 10 is produced will be described, but in actuality, aplurality of high-frequency signal transmission lines 10 are producedsimultaneously by stacking and cutting large-sized dielectric sheets.

Prepared first are dielectric sheets 18 a to 18 c made of athermoplastic resin and copper-foiled entirely on their top surfaces.The copper-foiled top surfaces of the dielectric sheet 18 a to 18 c aresmoothened, for example, by galvanization for rust prevention. Thedielectric sheets 18 a to 18 c are sheets of liquid crystal polymerhaving a thickness of from about 50 μm to about 150 μm. The thickness ofthe copper foil is from about 10 μm to about 20 μm.

Next, external terminals 16 a and 16 b and a ground conductor 22, asshown in FIG. 2, are formed on the top surface of the dielectric sheet18 a by photolithography. Specifically, resists are printed on thecopper foil of the dielectric sheet 18 a in the same shapes as theexternal terminals 16 a and 16 b and the ground conductor 22 shown inFIG. 2. Then, any portions of the copper foil that are not coated withthe resists are removed by etching the copper foil. Thereafter, theresists are removed. In this manner, the external terminals 16 a and 16b and the ground conductor 22, as shown in FIG. 2, are formed on the topsurface of the dielectric sheet 18 a.

Next, a signal line 20, as shown in FIG. 2, is formed on the top surfaceof the dielectric sheet 18 b by photolithography. Further, a groundconductor 24, as shown in FIG. 2, is formed on the top surface of thedielectric sheet 18 c by photolithography. These photolithographic stepsare essentially the same as the steps of forming the external terminals16 a and 16 b and the ground conductor 22 by photolithography, andtherefore, any descriptions thereof will be omitted.

Next, through-holes are provided in the dielectric sheets 18 a and 18 bby laser beam irradiation on their bottom surfaces where via-holeconductors b1, b2, and B1 to B4 are to be formed. Thereafter, thethrough-holes provided in the dielectric sheets 18 a and 18 b are filedwith a conductive paste.

Next, the dielectric sheets 18 a to 18 c are stacked in this order, fromthe positive side to the negative side in the z-axis direction, suchthat the ground conductor 22, the signal line 20, and the groundconductor 24 form a stripline structure. Thereafter, the dielectricsheets 18 a to 18 c are heated and pressed from both the positive andnegative sides in the z-axis direction and thus softened to be bondedand integrated together, and further, the conductive paste filling thethrough-holes is solidified so that the via-hole conductors b1, b2, andB1 to B4, as shown in FIG. 2, as formed. Note that the dielectric sheets18 a to 18 c may be integrated, for example, by using an epoxy resinadhesive, rather than by the heating and pressing process.Alternatively, the via-hole conductors b1, b2, and B1 to B4 may beformed by providing through-holes in the dielectric sheets 18 a to 18 chaving been integrated and filling the through-holes with a conductivepaste or plating the through-holes.

Next, a resin (resist) paste is applied to the top surface of thedielectric sheet 18 a, thus forming a protective layer 14 thereon.Lastly, connectors 100 a and 100 b are respectively mounted on the topsurfaces of the connecting portions 12 b and 12 c by solder. As aresult, the high-frequency signal transmission line 10 as shown in FIG.1 is obtained.

The high-frequency signal transmission line 10 and the electronic device200 render it possible to reduce insertion loss. More specifically, asfor the high-frequency signal line described in InternationalPublication No. 2012/074101, the entire signal line generates anessentially uniform characteristic impedance (e.g., 50Ω).

On the other hand, the signal line 20 includes the sections A1 and A2.The section A1 generates a characteristic impedance greater than orequal to the characteristic impedance Z1 (e.g., 50Ω) generated at theend of the signal line 20 on the negative side in the x-axis direction,and includes the end of the signal line 20 on the positive side in thex-axis direction. Further, the section A2 generates a characteristicimpedance less than the characteristic impedance Z1, and is adjacent tothe section A1. That is, in a portion of the signal line 20 of thehigh-frequency signal transmission line 10 (i.e., the section A2), thesignal line 20 generates a characteristic impedance less than thecharacteristic impedance Z1 generated at the end of the signal line 20on the negative side in the x-axis direction. Accordingly, thehigh-frequency signal transmission line 10 achieves reduced transmissionloss in the section A2 compared to the high-frequency signal linedescribed in International Publication No. 2012/074101. Thus, thehigh-frequency signal transmission line 10 can reduce insertion losscompared to the high-frequency signal line described in InternationalPublication No. 2012/074101.

Furthermore, to reduce the transmission loss in the section A2, thehigh-frequency signal transmission line 10 is designed such that thecharacteristic impedance generated in the section A2 is less than thecharacteristic impedance Z1 generated at the end of the signal line 20on the positive side in the x-axis direction. More specifically, to thisend, the high-frequency signal transmission line 10 is designed suchthat the width w2 of the signal line 20 in the section A2 is greaterthan the width w1 of the signal line 20 in the section A1. As a result,the resistance of the signal line 20 in the section A2 is reduced. Thus,conductor loss to be generated when a high-frequency signal istransmitted through the section A2 is reduced, resulting in reducedinsertion loss in the high-frequency signal transmission line 10.

Furthermore, in the high-frequency signal transmission line 10, thewidth of the signal line 20 changes continuously in the impedancechanging section a1, and therefore, the characteristic impedance of theimpedance changing section a1 changes continuously as well. Thus, thecharacteristic impedance generated in the impedance changing section a1is prevented from changing abruptly, thus preventing high-frequencysignal reflection in the impedance changing section a1.

Furthermore, the high-frequency signal transmission line 10 achievesreduced insertion loss also for the reasons stated below. Specifically,it is conceivable that the high-frequency signal transmission line isprovided with a balun, for example, in a portion corresponding to theimpedance changing section a1, in order to change the characteristicimpedance generated in the signal line. However, the balun causestransmission loss, resulting in increased insertion loss in thehigh-frequency signal transmission line.

The characteristic impedance of the high-frequency signal transmissionline 10 is generated by the signal line 20, the ground conductors 22 and24, and the dielectric element 12. Accordingly, the high-frequencysignal transmission line 10 does not use a circuit, such as a balun,which does not include the signal line 20, the ground conductors 22 and24, and the dielectric element 12, for the purpose of changing thecharacteristic impedance generated in the impedance changing section a1.Thus, the high-frequency signal transmission line 10 achieves reducedinsertion loss.

Furthermore, the length of the impedance changing section a1 is set tobe greater than or equal to about ⅕ or about ¼ of the wavelength of thehigh-frequency signal to be transmitted through the signal line 20, thuspreventing the impedance changing section a1 from functioning as anincommensurate element. That is, high-frequency signal reflection in theimpedance changing section a1 is prevented. As a result, the insertionloss in the high-frequency signal transmission line 10 is reduced. Notethat there is no upper limit to the length of the impedance changingsection a1. However, in the case where the high-frequency signaltransmission line 10 is used in the electronic device 200, thehigh-frequency signal transmission line 10 is required to have anappropriate length. Therefore, from the viewpoint of practical use, theupper limit to the length of the impedance changing section a1 ispreferably about twice the wavelength of the high-frequency signal to betransmitted through the signal line 20, for example.

First Modification

Hereinafter, a high-frequency signal transmission line 10 a according toa first modification of a preferred embodiment of the present inventionwill be described with reference to the drawings. FIG. 7A is an explodedview of a dielectric element 12 of the high-frequency signaltransmission line 10 a.

The high-frequency signal transmission line 10 a is different from thehigh-frequency signal transmission line 10 in that the signal line 20includes sections A1, A2, and A3. More specifically, in thehigh-frequency signal transmission line 10 a, the section A2 does notinclude the end of the signal line 20 on the positive side in the x-axisdirection. Instead, the section A3 includes the end of the signal line20 on the positive side in the x-axis direction, and is adjacent to thesection A2. That is, the section A3 extends toward the positive side inthe x-axis direction starting from the end of the section A2 on thepositive side in the x-axis direction. Moreover, the section A3 is anuninterrupted section generating a characteristic impedance greater thanor equal to a characteristic impedance Z4 (e.g., 50Ω) generated at theend of the signal line 20 on the positive side in the x-axis direction.In the present preferred embodiment, the characteristic impedancegenerated in the section A3 is equal or substantially equal to thecharacteristic impedance Z4.

The section A2 generates a characteristic impedance less than either thecharacteristic impedance Z1 or Z4, and is adjacent to both the sectionsA1 and A3. That is, the section A2 is sandwiched between the sections A1and A3. In addition, the section A2 is longer than either the section A1or A3. In the present preferred embodiment, the section A2 is longerthan a combined length of the sections A1 and A3.

Furthermore, the section A2 includes impedance changing sections a1 anda3 and an impedance unchanging section a2. The impedance changingsection a1 is adjacent to the section A1, and extends toward thepositive side in the x-axis direction starting from the end of thesection A1 on the positive side in the x-axis direction. Thecharacteristic impedance generated in the impedance changing section a1decreases toward the farthest end from the section A1 (i.e., toward theend on the positive side in the x-axis direction). The impedancechanging section a2 is adjacent to the impedance changing section a1,and extends toward the positive side in the x-axis direction startingfrom the end of the impedance changing section a1 on the positive sidein the x-axis direction. The impedance unchanging section a2 generatesan essentially uniform characteristic impedance Z10 (e.g., 30Ω). Theimpedance changing section a3 is adjacent to the impedance unchangingsection a2, and extends toward the positive side in the x-axis directionstarting from the end of the impedance unchanging section a2 on thepositive side in the x-axis direction. The characteristic impedancegenerated in the impedance changing section a3 increases toward thefarthest end from the section A2 (i.e., toward the end on the positiveside in the x-axis direction).

Furthermore, the length of the impedance changing section a3, as withthe length of the impedance changing section a1, is preferably greaterthan or equal to about ⅕, more preferably greater than or equal to about¼, of the wavelength of the high-frequency signal to be transmittedthrough the signal line 20. In addition, the length of the impedancechanging section a3, as with the length of the impedance changingsection a1, is preferably less than or equal to about twice thewavelength of the high-frequency signal to be transmitted through thesignal line 20.

In the high-frequency signal transmission line 10 a, the width of thesignal line 20 varies between the sections in order to generate thecharacteristic impedances described above. More specifically, the signalline 20 has the width w1 in both the sections A1 and A3. In theimpedance unchanging section a2, the signal line 20 has the width w2,which is greater than the width w1. In this manner, the signal line 20is relatively narrow in the sections A1 and A3, and therefore, the areaof the signal line 20 that overlaps the ground conductors 22 and 24 perunit length is relatively small. Accordingly, in the sections A1 and A3,the capacitance generated between the signal line 20 and the groundconductors 22 and 24 per unit length is relatively low, so that thecharacteristic impedance Z1 of the high-frequency signal transmissionline 10 a becomes relatively high. On the other hand, in the impedanceunchanging section a2, the signal line 20 is relatively wide, andtherefore, the area of the signal line 20 that overlaps the groundconductors 22 and 24 per unit length is relatively large. Accordingly,in the impedance unchanging section a2, the capacitance generatedbetween the signal line 20 and the ground conductors 22 and 24 per unitlength is relatively high, so that the characteristic impedance Z10 ofthe high-frequency signal transmission line 10 a becomes relatively low.

Furthermore, the signal line 20 has the width w1 at the end of theimpedance changing section a1 on the negative side in the x-axisdirection, and the signal line 20 has the width w2 at the end of theimpedance changing section a1 on the positive side in the x-axisdirection. That is, the width of the signal line 20 in the impedancechanging section a1 increases from the negative side in the x-axisdirection toward the positive side (i.e., toward the farthest end fromthe section A1). In addition, the width of the signal line 20 in theimpedance changing section a1 changes continuously. As a result, thecharacteristic impedance generated in the impedance changing section a1decreases continuously from the characteristic impedance Z1 to thecharacteristic impedance Z10 from the negative side in the x-axisdirection toward the positive side.

Furthermore, the signal line 20 has the width w2 at the end of theimpedance changing section a3 on the negative side in the x-axisdirection, and also has the width w1 at the end of the impedancechanging section a3 on the positive side in the x-axis direction. Thatis, the width of the signal line 20 in the impedance changing section a3decreases from the negative side in the x-axis direction toward thepositive side (i.e., toward the farthest end from the section A2). Inaddition, the width of the signal line 20 in the impedance changingsection a3 changes continuously. As a result, the characteristicimpedance generated in the impedance changing section a3 increasescontinuously from the characteristic impedance Z10 to the characteristicimpedance Z4 from the negative side in the x-axis direction toward thepositive side.

The characteristic impedance of the high-frequency signal transmissionline 10 a is generated by the signal line 20, the ground conductors 22and 24, and the dielectric element 12. Accordingly, the high-frequencysignal transmission line 10 does not use a circuit, such as a balun,which does not include the signal line 20, the ground conductors 22 and24, and the dielectric element 12, for the purpose of changing thecharacteristic impedance generated in the impedance changing sections a1and a3.

The high-frequency signal transmission line 10 a having such a structurealso achieves the same effects as those achieved by the high-frequencysignal transmission line 10.

The present inventor carried out a computer simulation to be describedbelow in order to demonstrate that the high-frequency signaltransmission line 10 a achieves reduced insertion loss and also in orderto determine an appropriate length of the impedance changing section a3.Specifically, the present inventor made a high-frequency signaltransmission line 10 a as a first model and a high-frequency signaltransmission line according to a comparative example as a second model.The high-frequency signal transmission line according to the comparativeexample includes a signal line with a uniform width to generate auniform characteristic impedance. The relationship of insertion loss(IL) and return loss (RL) over frequency was studied for both the firstmodel and the second model. Insertion loss is a value for the ratio ofoutput signal power to input signal power. Return loss is a value forthe ratio of reflected signal power to input signal power. FIG. 7B is agraph showing the simulation results where the vertical axis representsattenuation and the horizontal axis represents frequency.

It can be appreciated from FIG. 7B that the first model has lessinsertion loss than the second model for the frequency band of from 1.8GHz upward. Accordingly, it can be appreciated that the high-frequencysignal transmission line 10 a achieves reduced insertion loss.

The difference in insertion loss between the first model and the secondmodel is small around 3.3 GHz, as shown in FIG. 7B. The reason for thisis that return loss in the first model increases around 3.3 GHz. On theother hand, the difference in insertion loss between the first model andthe second model increases around 2.5 GHz and also around 4.7 GHz, asshown in FIG. 7B. The reason for this is that return loss in the firstmodel decreases around 2.5 GHz and also around 4.7 GHz. Therefore, thehigh-frequency signal transmission line 10 a is designed such that theposition of the dip in return loss for the first model in FIG. 7Bcorresponds to the frequency of the high-frequency signal to betransmitted through the signal line 20.

Furthermore, it can be appreciated from FIG. 7B that the first model hasmore insertion loss than the second model for the frequency band of from1.8 GHz downward and less insertion loss for the frequency band higherthan 1.8 GHz. The cause of such a phenomenon is due to the relationshipbetween the length of the impedance changing sections a1 and a3 and thehigh-frequency signal transmitted through the signal line 20. Morespecifically, when the length of the impedance changing sections a1 anda3 is an integral multiple of ¼ of the wavelength of the high-frequencysignal to be transmitted through the signal line 20, signal reflectionin the impedance changing sections a1 and a3 is prevented, resulting inreduced return loss in the first model. Accordingly, the length of theimpedance changing sections a1 and a3 is preferably greater than orequal to about ¼, more preferably an integral multiple of ¼, of thewavelength of the high-frequency signal to be transmitted through thesignal line 20. Note that in the present preferred embodiment, thelength of the impedance changing sections a1 and a3 is preferably set atabout ¼ of the wavelength of a 2.5-GHz high-frequency signal (i.e., thelength is set at the wavelength of a high-frequency signal at about 10GHz), for example. Accordingly, insertion loss in the first modeldecreases at about 2.5 GHz, for example.

Furthermore, the high-frequency signal transmission line 10 a achievesreduced insertion loss by increasing the width of the signal line 20 inthe section A2. The advantage of reducing insertion loss by increasingthe width of the signal line 20 in the section A2 is cancelled out at afrequency of 1.8 GHz by the disadvantage of increasing return loss dueto the length of the impedance changing sections a1 and a3 being shorterthan about ¼ of the wavelength of the high-frequency signal to betransmitted through the signal line 20. As described earlier, the lengthof the impedance changing sections a1 and a3 is preferably set at thewavelength of the high-frequency signal at about 10 GHz, for example. Inother words, the length of the impedance changing sections a1 and a3 ispreferably set at about ⅕ of the wavelength of the 1.8-GHzhigh-frequency signal, for example. Therefore, the length of theimpedance changing sections a1 and a3 may be set to be greater than orequal to about ⅕ of the wavelength of the high-frequency signal to betransmitted through the signal line 20.

Second Modification

Hereinafter, a high-frequency signal transmission line 10 b according toa second modification of a preferred embodiment of the present inventionwill be described with reference to the drawings. FIG. 8A is an explodedview of a dielectric element 12 of the high-frequency signaltransmission line 10 b. FIG. 8B is a cross-sectional structure view ofthe high-frequency signal transmission line 10 b taken along line A-A inFIG. 8A. FIG. 8C is a graph showing the characteristic impedance of thehigh-frequency signal transmission line 10 b. In FIG. 8C, the verticalaxis represents the characteristic impedance and the horizontal axisrepresents the x-coordinate.

The high-frequency signal transmission line 10 b is different from thehigh-frequency signal transmission line 10 in the following threepoints. The first difference is that the width of the signal line 20 isuniform. The second difference is that the ground conductor 24 isprovided with openings 30 a, 30 b, 30 c, 30 d, 30 e, and 30 f. The thirddifference is that the signal line 20 is provided closer to the groundconductor 24 than to the ground conductor 22.

The high-frequency signal transmission line 10 b will be described belowmainly focusing on these differences.

As for the first difference, the signal line 20 has a uniform width w3,as shown in FIG. 8A. The width w3 is greater than either the width w1 orw2.

As for the second difference, the ground conductor 24 is provided withthe openings 30 a to 30 f arranged along the signal line 20, as shown inFIG. 8A. The openings 30 a to 30 f, when viewed in a plan view in thez-axis direction, overlap the signal line 20, and are arranged in thisorder from the negative side to the positive side in the x-axisdirection. The openings 30 a to 30 f are circular. The openings 30 a and30 b have the same diameter, and are the largest in diameter among allof the openings 30 a to 30 f. Moreover, the openings 30 c to 30 f are indescending order of diameter. Accordingly, the openings 30 a and 30 bare the largest in overlapping area with the signal line 20 among all ofthe openings 30 a to 30 f. Moreover, the openings 30 c to 30 f are indescending order of overlapping area with the signal line 20.

As for the third difference, the signal line 20 is provided closer tothe ground conductor 24 than to the ground conductor 22, as shown inFIG. 8B.

In the high-frequency signal transmission line 10 b, the openings 30 ato 30 f are provided so that the section of the ground conductor 24 withthe openings 30 a to 30 f is smaller in area facing the signal line 20than the rest of the ground conductor 24. Accordingly, the capacitancegenerated between the signal line 20 and the ground conductor 24 islower in the section with the openings 30 a to 30 f than in the rest ofthe ground conductor 24. As a result, the characteristic impedancegenerated in the section with the openings 30 a to 30 f is higher thanin the rest of the ground conductor 24.

Furthermore, the openings 30 a and 30 b are the largest in diameteramong all of the openings 30 a to 30 f. In addition, the openings 30 cto 30 f are in descending order of diameter. The openings 30 a and 30 bare the largest in overlapping area with the signal line 20 among all ofthe openings 30 a to 30 f. In addition, the openings 30 c to 30 f are indescending order of overlapping area with the signal line 20.Accordingly, the characteristic impedance of the high-frequency signaltransmission line 10 b changes as shown in FIG. 8C. More specifically,in the section with the openings 30 a and 30 b, the characteristicimpedance of the high-frequency signal transmission line 10 b repeatedlyincreases and decreases in the range above the characteristic impedanceZ1. In the section with the openings 30 c to 30 f, the characteristicimpedance of the high-frequency signal transmission line 10 b decreasesfrom the characteristic impedance Z1 to the characteristic impedance Z10while repeatedly increasing and decreasing. The characteristic impedanceof the high-frequency signal transmission line 10 b is thecharacteristic impedance Z10 in the section without openings.

In the high-frequency signal transmission line 10 b, the sectionstretching from the end of the signal line 20 on the negative side inthe x-axis direction to the end of the opening 30 c on the negative sidein the x-axis direction corresponds to the section A1, as shown in FIG.8A. Moreover, the section stretching from the end of the opening 30 c onthe negative side in the x-axis direction to the end of the opening 30 fon the positive side in the x-axis direction corresponds to theimpedance changing section a1. In addition, the section stretching fromthe end of the opening 30 f on the positive side in the x-axis directionto the end of the signal line 20 on the positive side in the x-axisdirection corresponds to the impedance unchanging section a2.

Furthermore, the high-frequency signal transmission line 10 b isprovided with the openings 30 a to 30 f so that the capacitancegenerated between the signal line 20 and the ground conductor 24 isreduced. As a result, the characteristic impedance of the high-frequencysignal transmission line 10 b might become excessively low in thesection with the openings 30 a to 30 f. Therefore, in the high-frequencysignal transmission line 10 b, when compared to the high-frequencysignal transmission line 10, the distance between the signal line 20 andthe ground conductor 24 is reduced, and the width of the signal line 20is increased.

The high-frequency signal transmission line 10 b having such a structureachieves the same effects as those achieved by the high-frequency signaltransmission line 10.

Furthermore, the high-frequency signal transmission line 10 b isrendered thinner while achieving reduced insertion loss. Morespecifically, the ground conductor 24 is provided with the openings 30 ato 30 f so that the distance between the signal line 20 and the groundconductor 24 can be reduced, thus rendering the high-frequency signaltransmission line 10 b thinner. However, providing the ground conductor24 with the openings 30 a to 30 f generates radiation loss, resulting inincreased insertion loss in the high-frequency signal transmission line10 b. Radiation loss refers to loss generated by the signal line 20radiating noise through the openings to the outside of thehigh-frequency signal transmission line 10 b.

Therefore, the openings 30 c to 30 f are provided in the impedancechanging section a1 in descending order of area toward the farthest endfrom the section A1. Accordingly, the radiation loss in the impedancechanging section a1 decreases toward the farthest end from the sectionA1. Moreover, the radiation loss is significantly reduced or minimizedin the impedance unchanging section a2 because no openings are providedin the impedance unchanging section a2. That is, the generation ofradiation loss is significantly reduced or prevented in the section A2.In this manner, the high-frequency signal transmission line 10 b isprovided with the openings 30 a to 30 f, so that the high-frequencysignal transmission line 10 b is rendered thinner, and further, theopenings 30 a to 30 f are provided in various sizes so as to reduceradiation loss in the section A2. Thus, the high-frequency signaltransmission line 10 b is rendered thinner while achieving reducedinsertion loss.

Furthermore, in the high-frequency signal transmission line 10 b, thesignal line 20 is wider, and therefore, the conductor loss generatedwhen the signal line 20 transmits a high-frequency signal therethroughis reduced. Thus, the high-frequency signal transmission line 10 bachieves reduced insertion loss.

Furthermore, the high-frequency signal transmission line 10 b allows thesignal line 20 and the ground conductor 24 to be positioned closer toeach other, so that the dielectric element 12 is rendered thinner. Thus,the high-frequency signal transmission line 10 b is able to be bentreadily.

Third Modification

Hereinafter, a high-frequency signal transmission line 10 c according toa third modification of a preferred embodiment of the present inventionwill be described with reference to the drawings. FIG. 9A is an explodedview of a dielectric element 12 of the high-frequency signaltransmission line 10 c. FIG. 9B is a graph showing the characteristicimpedance of the high-frequency signal transmission line 10 c. For thecross-sectional structure view of the high-frequency signal transmissionline 10 c, FIG. 8C will be referenced.

The high-frequency signal transmission line 10 c is different from thehigh-frequency signal transmission line 10 in the following threepoints. The first difference is that the signal line 20 becomes widerfrom the negative side in the x-axis direction toward the positive side.The second difference is that the ground conductor 24 is provided with aplurality of openings 30. The third difference is that the signal line20 is provided closer to the ground conductor 24 than to the groundconductor 22.

The high-frequency signal transmission line 10 c will be described belowmainly focusing on these differences.

As for the first difference, the signal line 20 continues to becomewider from the negative side in the x-axis direction toward the positiveside, as shown in FIG. 9A.

As for the second difference, the ground conductor 24 is provided withthe openings 30 arranged along the signal line 20, as shown in FIG. 9A.The openings 30 are in the form of rectangles overlapping the signalline 20 when viewed in a plan view in the z-axis direction. Accordingly,the ground conductor 24 preferably has a ladder-shaped configuration.Each portion of the ground conductor 24 that is located between theopenings 30 at opposite ends in the x-axis direction will be referred tobelow as a “bridge portion 60”.

As for the third difference, the signal line 20 is provided closer tothe ground conductor 24 than to the ground conductor 22, as shown inFIG. 8B.

In the high-frequency signal transmission line 10 c, the capacitancegenerated between the signal line 20 and the ground conductor 24 islower at the openings 30 than at the bridge portions 60. Accordingly,the characteristic impedance of the high-frequency signal transmissionline 10 c is higher at the openings 30 than at the bridge portions 60.The signal line 20 alternatingly overlaps the openings 30 and the bridgeportions 60. Therefore, the characteristic impedance of thehigh-frequency signal transmission line 10 c repeatedly increases anddecreases, as shown in FIG. 9B.

However, the signal line 20 becomes wider from the negative side in thex-axis direction toward the positive side. Accordingly, the capacitancegenerated between the signal line 20 and the ground conductor 24increases from the negative side in the x-axis direction toward thepositive side. Therefore, the characteristic impedance of thehigh-frequency signal transmission line 10 c becomes lower from thenegative side in the x-axis direction toward the positive side whilerepeatedly increasing and decreasing, as shown in FIG. 9B.

In the high-frequency signal transmission line 10 c, the characteristicimpedance of the high-frequency signal transmission line 10 c is thecharacteristic impedance Z1 at the end of the signal line 20 on thenegative side in the x-axis direction, and increases slightly toward thepositive side in the x-axis direction, as shown in FIG. 9B. The sectionA1 continues from the end of the signal line 20 on the negative side inthe x-axis direction up to a point at which the characteristic impedanceinitially reaches the characteristic impedance Z1 on the positive sidein the x-axis direction relative to the end of the signal line 20 on thenegative side in the x-axis. That is, the section A1 is an uninterruptedsection in which the characteristic impedance Z1 or higher continues tobe generated. In the present preferred embodiment, the section A1stretches from the end of the signal line 20 on the negative side in thex-axis direction to the farthermost one of the bridge portions 60 on thenegative side in the x-axis direction.

Furthermore, the section A2 is a section stretching from the end of thesection A1 on the positive side in the x-axis direction to the end ofthe signal line 20 on the positive side in the x-axis direction. Notethat the characteristic impedance generated in the section A2essentially decreases. Accordingly, the high-frequency signaltransmission line 10 c has no impedance unchanging section a2.Therefore, in the high-frequency signal transmission line 10 c, thesection A2 and the impedance changing section a1 are equivalent to eachother.

The high-frequency signal transmission line 10 c having such a structureachieves the same effects as those achieved by the high-frequency signaltransmission line 10.

Furthermore, the high-frequency signal transmission line 10 c allows thesignal line 20 and the ground conductor 24 to be positioned closer toeach other, so that the dielectric element 12 is rendered thinner. Thus,the high-frequency signal transmission line 10 b is able to be bentreadily.

Incidentally, the present inventor carried out a computer simulation tobe described below in order to demonstrate that the high-frequencysignal transmission line 10 c achieves reduced insertion loss.Specifically, the present inventor made third through fifth models to bedescribed below.

The third model has the configuration of the high-frequency signaltransmission line 10 c. Note that the characteristic impedance Z1 is50Ω, and the characteristic impedance Z4 is 30Ω.

The fourth model has the configuration of the high-frequency signal linedescribed in International Publication No. 2012/074101. Thecharacteristic impedance of this high-frequency signal line is constantat 50Ω.

The fifth model is the high-frequency signal line described inInternational Publication No. 2012/074101 with a balun. This signal linehas a characteristic impedance of 50Ω at one end and a characteristicimpedance of 30Ω at the other end.

The present inventor studied the relationship of reflectioncharacteristic and pass characteristic over frequency for the thirdthrough fifth models. FIG. 10A is a graph showing the reflectioncharacteristic of the third model. FIG. 10B is a graph showing the passcharacteristic of the third model. FIG. 11A is a graph showing thereflection characteristic of the fourth model. FIG. 11B is a graphshowing the pass characteristic of the fourth model. FIG. 12A is a graphshowing the reflection characteristic of the fifth model. FIG. 12B is agraph showing the pass characteristic of the fifth model. Passcharacteristic is a value for the ratio of output signal intensity atthe external terminal 16 b to input signal intensity at the externalterminal 16 a. Reflection characteristic is a value for the ratio ofreflected signal intensity at the external terminal 16 a to input signalintensity at the external terminal 16 a.

As for the third model, the reflection characteristic is best at 2.4GHz. The pass characteristic of the third model is −0.477 dB at 2.4 Hz.

As for the fourth model, the reflection characteristic is best at 2.4GHz. The pass characteristic of the fourth model is −0.504 dB at 2.4GHz.

As for the fifth model, the reflection characteristic is best at 2.4GHz. The pass characteristic of the fifth model is −1.307 dB at 2.4 GHz.

From the foregoing, it can be appreciated that the pass characteristicof the third model at 2.4 GHz is better than the pass characteristic ofeach of the fourth and fifth models at 2.4 GHz. Accordingly, it can beappreciated that the high-frequency signal transmission line 10 cachieves reduced insertion loss compared to both the high-frequencysignal line described in International Publication No. 2012/074101 andthe high-frequency signal line with a balun.

Other Preferred Embodiments

The present invention is not limited to the high-frequency signaltransmission lines 10 and 10 a to 10 c, and variations can be madewithin the spirit and scope of the present invention.

The high-frequency signal transmission lines 10 and 10 a to 10 c mayonly have either one of the ground conductors 22 and 24. That is, thehigh-frequency signal transmission lines 10 and 10 a to 10 c may bemicro-stripline structures.

Moreover, the dielectric element 12 does not have to be a stack oflayers. For example, the dielectric element 12 may be a cable having acircular cross section as with coaxial cables. In such a case, anexternal conductor having an annular cross section is provided so as tosurround the signal line.

Furthermore, the high-frequency signal transmission line 10 c does nothave to be provided with the openings 30. In such a case, thecharacteristic impedance of the high-frequency signal transmission line10 c decreases linearly from the characteristic impedance Z1 to thecharacteristic impedance Z4. Accordingly, the section A1 does notcontinue from the end of the signal line 20 on the negative side in thex-axis direction. In addition, the section A2 spans the entire signalline 20 excluding the end on the negative side in the x-axis direction.Moreover, the impedance changing section a2 does not exist.

The configurations of the high-frequency signal transmission lines 10and 10 a to 10 c may be combined arbitrarily.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A high-frequency signal transmission linecomprising: an element; a linear signal line provided at the element andincluding a first end and a second end; and at least one groundconductor provided at the element and extending along the signal line;wherein the element includes stacked insulating layers; the groundconductor is positioned opposite to the signal line with the insulatinglayer positioned therebetween; the ground conductor is a contiguousconductor; the signal line, the ground conductor, and the elementgenerate a characteristic impedance; a characteristic impedance of thesecond end is less than a characteristic impedance of the first end; thesignal line includes a first section and a second section, the firstsection being an uninterrupted section generating a characteristicimpedance greater than or equal to a first characteristic impedance atthe first end and including the first end, the second section generatinga characteristic impedance less than the first characteristic impedanceand being adjacent to the first section; the second section is longerthan the first section; and the signal line is wider in the secondsection than in the first section.
 2. The high-frequency signaltransmission line according to claim 1, wherein the second sectionincludes a characteristic impedance changing section adjacent to thefirst section and having a characteristic impedance decreasing toward afarthest end from the first section.
 3. The high-frequency signaltransmission line according to claim 2, wherein the second sectionincludes a characteristic impedance unchanging section adjacent to thecharacteristic impedance changing section and generating a secondcharacteristic impedance that is uniform or substantially uniform. 4.The high-frequency signal transmission line according to claim 3,wherein the characteristic impedance changing section has a lengthgreater than or equal to about ⅕ of a wavelength of a high-frequencysignal to be transmitted through the signal line.
 5. The high-frequencysignal transmission line according to claim 2, wherein the signal linehas a width increasing in the characteristic impedance changing sectiontoward the farthest end from the first section.
 6. An electronic devicecomprising: a high-frequency signal transmission line; and a housingaccommodating the high-frequency signal transmission line; wherein, thehigh-frequency signal transmission line includes: an element; a linearsignal line provided at the element and including a first end and asecond end; and at least one ground conductor provided at the elementand extending along the signal line; the element includes stackedinsulating layers; the ground conductor is positioned opposite to thesignal line with the insulating layer positioned therebetween; theground conductor is a contiguous conductor; the signal line, the groundconductor, and the element generate a characteristic impedance; acharacteristic impedance of the second end is less than a characteristicimpedance of the first end; the signal line includes a first section anda second section, the first section being an uninterrupted sectiongenerating a characteristic impedance greater than or equal to a firstcharacteristic impedance at the first end and including the first end,the second section generating a characteristic impedance less than thefirst characteristic impedance and being adjacent to the first section;the second section is longer than the first section; and the signal lineis wider in the second section than in the first section.
 7. Theelectronic device according to claim 6, wherein the second sectionincludes a characteristic impedance changing section adjacent to thefirst section and having a characteristic impedance decreasing toward afarthest end from the first section.
 8. The electronic device accordingto claim 7, wherein the signal line has a width increasing in thecharacteristic impedance changing section toward the farthest end fromthe first section.
 9. The electronic device according to claim 7,wherein the second section includes a characteristic impedanceunchanging section adjacent to the characteristic impedance changingsection and generating a second characteristic impedance that is uniformor substantially uniform.
 10. The electronic device according to claim9, wherein the characteristic impedance changing section has a lengthgreater than or equal to about ⅕ of a wavelength of a high-frequencysignal to be transmitted through the signal line.